N-Methyl-2-pyrrolidone is an organic polar solvent excellent in heat stability and chemical stability and is therefore widely utilized as a chemical reaction medium, recrystallization medium, or a cleaning agent.
In particular, N-methyl-2-pyrrolidone is useful as a reaction solvent in the reaction between dihalogenated aromatic compounds and alkali metal sulfides for the synthesis of polyarylene sulfides as disclosed in JP-B 45-3368 (the term "JP-B" as used herein means an "examined published Japanese patent application"). It has been industrially employed as a polymerization solvent for obtaining polyphenylene sulfide (hereinafter abbreviated as PPS), one of the polyarylene sulfides.
The above-described process for producing PPS is frequently attended by by-production of thiophenol or diphenyl disulfide, and use of N-methyl-2-pyrrolidone containing the by-products thiophenol and/or diphenyl disulfide as a polymerization solvent results in serious reduction of molecular weight of the resulting polymers.
In addition, thiophenol or diphenyl disulfide is liable to form a complex compound with N-methyl-2pyrrolidone so that distillation of the spent solvent is not sufficient for separation of the thiophenol or diphenyl disulfide from N-methyl-2-pyrrolidone.
However, no study has been made as yet on purification of N-methyl-2-pyrrolidone containing thiophenol or diphenyl disulfide.